Interactions between molecules are responsible for a wide diversity of biological phenomena including protein structure, DNA base pairing, and the hydrophobic effect. Yet the fundamental nature of the hydrogen bond, perhaps the most biologically relevant, remains incompletely understood, based largely on decomposition methods developed and applied in the 1970s, using basis sets that were not well-suited to such studies. Moreover, the earlier work was restricted to the SCF level, neglecting completely the effects of electron correlation which are now known to be profound. The present project aims to dissect the correlation contribution to hydrogen bonding into a number of components; the SCF components will also be computed much more reliably than in the past. These components will be evaluated by a new hybrid procedure which combines the best features of intermolecular and Moller-Plesset supermolecular perturbation theory. The behavior of each of the components will be studied for a number of different chemical groups which commonly occur in biomolecules and for a range of different geometries. The ultimate goal of the project is a basic understanding of the nature of the H-bond. Model potentials will be developed to assist in ab initio calculations where it is not feasible to calculate correlation effects explicitly. The potentials will also find use in dynamics and Monte Carlo simulations of biomolecules in aqueous environment.